FIG. 1 is a cross-sectional view illustrating the basic construction of a multiplate friction clutch 10. FIG. 1 shows a clutch casing 1, a hub 2 as a counterpart element to which power is to be transmitted, and a piston 3 movable within the clutch casing 1. Designated at numerals 4 and 5 are separator plates and friction plates, respectively, which are arranged alternately side by side to make up a stack of friction plates. In the illustrated example, the separator plates 4 are in fitting engagement at outer circumferential portions thereof with spline grooves 11 in the clutch casing 1, and on the other hand, the friction plates 5 are in fitting engagement at inner circumferential portions thereof with spline grooves 21 in the hub 2.
When desired to engage the clutch, a hydraulic pressure is introduced into between the clutch casing 1 and the piston 3 through an oil hole 12. As a result, the piston 3 is pressed rightwards as viewed in the figure so that the stack of friction plates is pressed toward a backing plate 7. As the backing plate 7 is prevented from moving rightwards by a stopper ring 8, the clutch is brought into engagement. When desired to disengage the clutch, on the other hand, the hydraulic pressure is released. As a result, the piston 3 is allowed to return leftwards owing to the arrangement of a return spring 9.
Keeping in step with the recent move toward high-speed and high-power engines, it has become a practice to apply a cushioning resilient member to a piston on a surface thereof where the piston is brought into contact with an associated stack of friction plates. It was firstly contemplated to use a coned disk spring as a resilient member for relatively large loads. Specifically referring to FIG. 1, it was contemplated to form a holding groove 31 in a pressing wall of the piston 3 and to insert a coned disk spring in the holding groove 31.
In a high-torque-capacity clutch, however, an increase in the number of friction plates in a stack of friction plates leads to a greater overall clearance between the piston 3 and the backing plate 7. As the clutch is operated repeatedly, the wear of friction linings on the friction plates progresses, resulting in a still greater overall clearance. A potential problem, therefore, arises that the coned disk spring may fall off the holding groove when the clutch is not in operation. To avoid such a potential problem, it was proposed to form an annular protuberance on an edge of an opening of the holding groove and further, to form the coned disk spring in an elliptical shape (see JP-A-2004-144197).
When the holding groove is provided at the edge of its opening with the annular protuberance for the prevention of falling-off of the cushioning coned disk spring as mentioned above, the coned disk spring is required to ride over the annular protuberance upon mounting the coned disk spring on the piston. Formation of a cut in a coned disk spring may, however, structurally lead to a potential problem that, when pressed, the resulting coned disk spring may not be able to produce resiliency as much as it would do if the cut were not formed. The formation of such a cut further requires to form the coned disk spring in an elliptical shape and to fit the resulting elliptical coned disk spring in the holding groove while pressing the elliptical coned disk spring inwardly from both sides along a major axis into a shape close to a true circle. Moreover, after the mounting, the coned disk spring is in contact with the protuberance only at parts thereof in the direction of a minor axis.
As there is an ever-increasing demand for clutches of higher performance, it is required to mount a cushioning resilient member even on a clutch of relatively small torque capacity. In such a case, the use of a wave spring is suited. A wave spring allows to form a cut therein so that its handling is easier than a coned disk spring. The resulting wave spring with the cut formed therein is, however, accompanied by an inconvenience that under centrifugal force and vibrations, it has greater tendency of falling off the holding groove from the position of the cut.